NEW YORK (CNN) -- Longtime anchor Dan Rather will leave the "CBS Evening News" on March 9, the network said Tuesday, just months after Rather's use of questionable documents in a report critical of President Bush's National Guard service.

Rather, 73, said he will continue to work full time as a correspondent for the network's two "60 Minutes" programs as well as other assignments.

"I have always been and remain a 'hard news' investigative reporter at heart," Rather said in a statement. "I now look forward to pouring my heart into that kind of reporting full time."

CBS spokesman Gil Schwartz said that Rather's decision to leave "was independent of the findings of the panel looking into the '60 Minutes II' report on President Bush's National Guard service."

But Washington Post media reporter Howard Kurtz, host of CNN's "Reliable Sources," said he thought Rather's departure may be linked to the Bush National Guard story.

"It's pretty clear that Dan Rather faced a very unpalatable choice," Kurtz said. "... His contract had at least two more years to run. [Should he] step down now before the outside investigative report, commissioned by CBS News expected in the next few weeks about his botching -- and the network's botching -- of that story about President Bush's National Guard service? Rather [decided] today that it would be better for him to step down on his own terms."

On a September 8 segment of "60 Minutes II," Rather reported allegations that during the Vietnam era, Bush received special consideration to get into the Texas Air National Guard and subsequently did not fulfill his service obligations, including ignoring an order to get a required physical exam.

Among the evidence cited were four memos critical of the future president, purportedly written by Bush's then-squadron commander, Lt. Col. Jerry Killian, who died in 1984.

But immediately after the broadcast, the documents came under fire in media reports, with some document experts saying that they were produced with a computer word-processing program, not by a 1970s-era typewriter. Killian's former wife, son and secretary all questioned the validity of the memos, though his secretary said they did reflect his views at the time about Bush.

The documents were supplied by Bill Burkett, a retired Texas Army National Guard officer and longtime Bush critic

Early in the controversy, Rather and CBS News had insisted that the documents came from a "solid" source, that their contents were backed up by other reporting and that the memos had been authenticated by document experts. However, Rather conceded that CBS had only obtained photocopies of the documents, not the originals, which experts said would shed light on their authenticity.

On September 20, CBS announced it could no longer be sure the documents were genuine, and Rather apologized on the network's flagship "Evening News" broadcast.

In a statement on the documents, CBS News President Andrew Heyward said, "We should not have used them. That was a mistake, which we deeply regret."

Rather will leave his post 24 years to the day after he replaced Walter Cronkite as the network's main anchor. He said he also will step down as the program's managing editor.

"I have been lucky and blessed over these years to have what is, to me, the best job in the world and to have it at CBS News," Rather said. "Along the way, I've had the honor of working with some of the most talented, dedicated professionals in the world, and I'm appreciative of the opportunity to continue doing so in the years ahead."

Leslie Moonves, chairman of CBS, said Rather has "played a crucial role in keeping the American public informed."

"Dan's 24 years at the 'CBS Evening News' is the longest run of any evening news anchor in history and is a singular achievement in broadcast journalism," Moonves said in a written statement.

Heyward said Rather's "dedication to his craft and his remarkable skills as a reporter are legendary."

"He has symbolized the 'CBS Evening News' for nearly a quarter century," Heyward said in a written statement. "He'll continue to apply his talents to everything he does at CBS News."

There has been no official announcement about a replacement, but there has been speculation about CBS chief White House correspondent John Roberts and another CBS veteran, correspondent Scott Pelley.

Rather, a Texas-born journalist, first joined CBS in 1962 as the chief of its Southwest bureau in Dallas, Texas, from which he reported details of President Kennedy's assassination on November 22, 1963.

The Rather announcement comes just before the scheduled departure of another longtime broadcast network news anchor, Tom Brokaw, who has anchored the "NBC Nightly News" since 1983. Brian Williams will replace Brokaw.

Rather and Brokaw, along with "ABC World News Tonight" anchor Peter Jennings, have dominated TV broadcast networks' newcasts for more than two decades. Recently, Rather has usually ranked third in weekly ratings among the three network evening news shows.

Rather scored a journalistic coup before the start of the Iraq war, interviewing Iraqi President Saddam Hussein in February 2003 before the U.S.-led invasion.